= Lettuce red and far-red germination experiment
The 1952 experiment used Grand Rapids <lettuce> <seeds> on wetted blotters in covered dishes. After 16 hours in darkness, seven lots of 200 received successive alternating exposures: one minute of red <light> at $580$–$680\,\mathrm{nm}$, then four minutes above $700\,\mathrm{nm}$, with one lot removed after each exposure. Treated lots and a dark <scientific control> remained at $20\,{}^\circ\mathrm{C}$ for two days; germinated <seeds> were counted. Separate wavelength tests used a two-glass-prism <spectrograph>.<A Reversible Photoreaction Controlling Seed Germination by Harry Alfred Borthwick and others>[A Reversible Photoreaction Controlling Seed Germination, pp. 662–664: preparation, spectrograph and alternating-exposure protocol]{ref}
Table 1 shows alternating promotion and suppression according to the final exposure. Red <light> could restore promotion after far-red <light>; suppression was incomplete. This reversibility supports a reversible sensing state rather than simple cumulative illumination. The paper did not isolate or identify the pigment; the response concerns this <lettuce> preparation and timing. Its readout was a two-day <germination> count, not subsequent <seedling> performance.<A Reversible Photoreaction Controlling Seed Germination by Harry Alfred Borthwick and others>[A Reversible Photoreaction Controlling Seed Germination, p. 663 pigment-identification limitation and p. 665 Table 1]{ref}
The modern <phytochrome> description distinguishes forms $P_r$ and $P_{fr}$ that interconvert under red and far-red <light>. This provides a concrete example of <signal transduction>: <light> changes a sensing state that affects development. The response should be distinguished from <photosynthesis>, which supplies chemical resources.<Biology 2e by Mary Ann Clark and others>[Biology 2e, §30.6 The Phytochrome System and the Red/Far-Red Response, Figure 30.38: interconversion and cellular response]{ref}
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